documentation about grub.cfg

[c.buhtz]

Hello,

I am looking for a documentation about grub.cfg (Grub2) and the
possible keywords/commands/values I can use in there. I didn't find one
on your website.

I only find tutorials using update-grub and things like that. But I
need to write grub.cfg by myself.

Background of my question:
I want to manipulate the Grub2 installed on a USB-Stick. The current
running system (Ubuntu 14.04.3) is bootet from a hard disc - NOT from
the USB-Stick. I can install grub2 on the stick and see the menu when
booting from it.
But I want to manipulate that Grub2 - e.g. resolution, background
image, iso-boot, ...
And I only find help-sites describing update-grub, etc. But that
affects only the Grub from the current booted system (hda1) and not my
usb-stick (sdb).